1. Field of the Invention
The present invention generally relates to medical monitors: more specifically to electrical sensing apparatus for sensing body functions and still more specifically to an apnea respirometer utilizing acoustic impulses that are transmitted through the body.
2. Description of the Prior Art
Apnea measurement apparatus have been known in the art for some time, including electrical respirometers.
It is known that apnea is often measured with impedance respirometry. In general, a low current, medium frequency (20 KHz to 100 KHz) signal is passed through the thorax to sensors from which the impedance is calculated. The current path is selected such that variations in impedance will correspond to breathing motion.
This form of apnea measurement has certain disadvantages including potential interference with passive ECG measurements; probable reliance on ECG sensors; sensitivity to cardiac generated electrical signals and cardiac artifacts; and, sensitivity to ambient electrical noise and to direct electrical contact and current flow through the patient.
It is also known that apnea may be measured with airflow temperature sensing techniques, which techniques have the disadvantages, for example, of the airflow sensor blocking the nose or mouth, and sensitivity to ambient temperature and airflow conditions.
U.S. Pat. No. 5,220,922 teaches an ultrasonic medical motion monitoring device which bounces transduced sound waves off moving objects in the body, such as the heart, and reads the phase changes in the echo-return waves. This is similar to the prior art use of ultrasonic waves and carries the same disadvantages discussed above.
U.S. Pat. Nos. 4,197,856 and 4,122,427 also disclose ultrasonic body motion detectors.
U.S. Pat. No. 4,306,567 discloses a pressure sensor used to monitor body rhythmic functions by detecting pressure waves within a narrow bandwidth, the frequency being centered about 400 Hz for monitoring lung motion, and 45 Hz for monitoring cardiac activity.
U.S. Pat. No. 4,967,751 detects breathing rate with Doppler frequency shift by monitoring the cyclical movement of the thorax.